MIL-PRF-21038F
4.7.9 Life (see 3.1 and 3.12). Unless otherwise specified (see 3.1), transformers shall be subjected to five
life cycles a week for 12 weeks. Four of the cycles each week shall consist of 20 hours at oven temperature
equal to the specified maximum ambient temperature (see 3.1) for the transformers under test, and 4 hours at
room ambient temperature. The fifth cycle of the week shall be 68 hours at the specified maximum ambient
temperature (see 3.1) for the transformers under test, and 4 hours at room ambient temperature. When the
transformers are subjected to the maximum ambient temperature, they shall be applied with pulse test in
accordance with individual specification sheets, as specified in the electrical characteristics (see 4.7.3). After
the test, the transformers shall be examined for evidence of physical damage.
4.7.10 Vibration, high frequency (see 3.13). Transformers shall be tested in accordance with MIL-STD-202,
method 204, test condition B. Transformers shall be rigidly mounted by their normal mounting means.
4.7.11 Shock, specified pulse (see 3.14). Transformers shall be tested in accordance with MIL-STD-202,
method 213, test condition I. Transformers shall be rigidly mounted by their normal mounting means.
4.7.12 Thermal shock (see 3.15). Specimens shall be tested in accordance with MIL-STD-202, method 107,
test condition A, with the following exceptions: Number of cycles equals 10 (ten) and step 3 temperature to be
maximum operating temperature (see 3.1). In the event of no more than one unintentional test interruption
(power interruption or equipment failure) prior to the completion of the first five cycles, the test cycle shall be
repeated and the test continued. Unintentional interruptions occurring during the last cycle require a repeat of
the cycle plus an additional uninterrupted cycle. Any intentional interruption, or any unintentional interruption of
greater than 24 hours requires a complete retest.
4.7.13 Moisture resistance (see 3.16). Transformers shall be tested in accordance with MIL-STD-202,
method 106. The following details and exceptions shall apply:
a.
Mounting: Transformers shall be clamped in the trough of a 90 degree metallic V-block using a
metallic strap with a .075 inch (1.91 mm)-thick layer of conductive moisture-resistant resilient material,
having a resistivity of less than 1,000 ohms per centimeter, bonded to the surface of the strap next to
the transformer. The body of the transformer shall not extend beyond the extremities of the block or
resilient material. The surface of the V-block shall be free from contamination. The transformer leads
shall be so positioned that the distance between the leads and any point of the V-block shall be not
less than one half of the shortest base dimension. The metallic V-block shall be of non-corrosive
material. An alternative mounting method is shown in figure 1.
b.
Initial measurements: Not applicable.
c.
Polarization: Unless otherwise specified (see 3.1 and 6.2), the polarizing voltage shall be applied
during steps 1 through 6 between the coil winding (all leads connected together) and the metal strap to
the transformer assembly as specified in figure 1 or with the transformer strapped into a V-block.
When dielectric withstanding voltage is less than 100 volts rms, a 25 volt rms or a 25 volts dc may be
used. The polarizing voltage shall be positive with respect to the core and or the case.
d.
Final examinations: Upon completion of step 6 of the final cycle, transformers shall be removed from
the humidity chamber and shall be conditioned for 24 hours minimum in a test chamber at 85°C
±
5°C. After this conditioning period, transformers shall be examined for evidence of leakage and other
visible damage.
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